The process of soldering electronic circuitry involves placing and soldering leaded and leadless components on a printed circuit board. During the assembly process, components must be maintained in their proper positions prior to and during the soldering operation. In the case of leaded components, leads inserted into holes in the printed circuit board serve to aid in fixturing the component in place during the soldering operation. In leadless component soldering, there exists a need to fixture the components so that they remain in place and do not move about. This is generally accomplished by the use of an adhesive strategically placed underneath or alongside the component. Prior to assembly, solder is applied to the printed circuit board by stenciling or printing solder paste or by cladding the printed circuit board with solder. The adhesive is normally dispensed or deposited onto the printed circuit board, and the components are then placed in the adhesive. The adhesive is cured either thermally or with ultraviolet light. In each of these cases, a solder flux of some sort must be provided in order to facilitate sound metallurgical solder joints. A flux is normally formulated into the solder paste, or applied as a coating over the entire circuit board.
After soldering, flux residues are removed from the circuit board, typically by cleaning the soldered assembly in either an aqueous or organic cleaning solvent. The existence of an adhesive material underneath or adjacent to components provides a hindrance to cleaning and removing all traces of solder flux from beneath components. The standard industry practice to secure components in place is to use a thermosetting adhesive which cannot be removed after soldering. The cleaning operation is an expensive additional step and, in the case of organic solvents, utilizes material such as chlorofluorocarbons which are environmentally hazardous.
The prior art has attempted to utilize conventional soldering fluxes to hold components in place during the soldering operation. U.S. Pat. No. 4,720,324 discloses the use of solder flux coated over the entire surface of the printed circuit board and, while in a tacky state, the components are added. The tackiness or stickiness of the flux is purported to hold the component in place. While it is clear that conventional soldering flux does provide some degree of tackiness, this method does not function adequately. The resulting coating is too thin to act as a sufficient tacking agent and, when small, closely spaced components are soldered, the components tend to be drawn to one another by the surface tension of the flux and do not stay in their selected positions. Flood coating of the printed circuit board cannot be used with small, light components as they tend to swim or move about in the flux. Further, flood coating results in a waste of flux, as only a small portion of the board real estate is consumed by the solder pads. This increases the need for cleaning and the associated costs and hazards. Clearly, there exists a need for a process to provide a high efficiency, temporary fixturing means for components that also provides a fluxing action for the soldering operation. Such a process would be ideally compatible with printed circuit substrates having solid solder deposition (SSD) or clad solder surfaces, would be highly efficient, and would significantly reduce the need for cleaning, thereby aiding the environment.